The invention relates to a contact display panel circuitry in which there are phototransistors on two adjacent sides of the contact display panel and on the opposite sides there are diodes emitting infrared light to the transistors, and in which, under the control of shift registers, each phototransistor in turn is switched to the common line.
It is known that contact display panels have a structure in which the phototransistors are located on two adjacent sides of the panel and the diodes emitting infrared light are located on the sides opposite to these sides. The components are arranged so that there is always an infrared diode in alignment with a phototransistor on the opposite side. The imaginary lines interconnecting each pair of a phototransistor and an infrared diode form a grid. The diodes transmit infrared radiation, pulsed continuously or intermittently, to the phototransistors. The basic principle is that, when some point in the display panel is touched by a finger, the infrared radiation to the bases of the phototransistors in the contact line both in the X direction and the Y direction is cut off, whereupon the logic circuit observes the change in their collector or emitter current and is thus capable of computing the coordinates of the contact point.
There is known a circuitry, shown in FIG. 1, in which the phototransistors are coupled via an analog cmos switch to a common line, which leads to the logic circuit. The phototransistors 1 continuously receive supply voltage, and their emitter is coupled to switch K, which for its part is coupled to the common line c. The emitters are also coupled to the ground of the circuit by resistor R. Between the transistors 1 and the switches K there is additionally a differential capacitor C. Infrared diodes (not shown) transmit infrared radiation to the bases of the transistors 1, from which a certain emitter current and emitter voltage follow.
The transfer registers SR transmit at a certain frequency a control pulse, scanned, to each switch K, whereupon the switch K closes. From the emitter of the phototransistor, a pulsed signal, modulated by the infrared beam, is obtained. If a transistor is not receiving infrared radiation, the signal received from its emitter disappears or is considerably reduced. The logic, which also controls the shift registers SR, will now be capable of determining the phototransistors concerned and thus the contact point. The disadvantages of this prior-art circuitry include that a large number of resistors, capacitors and analog switching circuits or the like are needed, and arranging the necessary wiring on a printed circuit card is difficult.
Another known switching method is depicted in FIG. 2, in which the reference numerals are in applicable parts the same as in FIG. 1. In it multiplexer/decoders are used for switching each phototransistor 1 in turn to the line c leading to the common logic. The diodes emitting the infrared radiation which generates the base current of the phototransistors are not shown in the figure. The multiplexer/decoders are controlled by control signals (a, b, c) arriving along the control channel and by the selection signals of the multiplexer/ decoder. The emitter voltage of the phototransistors, which is the voltage effective across the resistor R, is applied via the differential capacitor to the multiplexer. The a, b, c signals of the control channel select in turn the emitter voltage of one phototransistor 1 at a time for being switched to the line c, from the voltage level change of which the logic circuit is capable of identifying the coordinates of the contact point. The disadvantages of this second prior-art method include that the control of the multiplexer/decoders requires a relatively large number of control lines and that the implementation (coding) of the control by means of multiplexer/decoders is more cumbersome than when using shift registers. In this circuitry, also, a large number of resistors and capacitors are used.